The problems associated with measuring flow rate in a multiphase fluid are well known. The techniques that can be applied to flows of a single phase such as systems including a turbine or the like turn out to be unsuitable in multiphase flows, and consequently it is often necessary to perform a plurality of measurements in order to be able to obtain a result that is accurate with respect to the volume flow rates of the various phases. In the oil and gas industry, and in comparable industries, it is common practice to measure multiphase flows, and in a production well, it is usual for the fluid products to comprise a mixture of oil, of water, and often of gas. Given that it is important to determine the volume flow rates of the individual phases at certain time intervals throughout the lifetime of the well, in order to determine whether any corrective action is required to improve or restore the productivity of the well, and also to determine exactly how much oil is being produced, various methods have been proposed in the prior art for measuring flow rates in multiphase flows. In conventional manner, when testing exploration wells, the fluids are sent to separators and the volumes of each phase are determined separately. However, that requires an installation that is voluminous and expensive and it does not make it possible to obtain information instantaneously about the production of the well.
Proposals have also been made to combine a venturi type device, a gamma ray densitometer, and a turbine meter (R. Franck et al., European Two Phase Flow Group Meeting, University of Strathclyde, Glasgow, Jun. 3-6, 1980). That system was developed for two-phase flows (water/steam) and is not capable of measuring thre-phase flows.
U.S. Pat. Nos. 4,856,344 and 4,974,542 describe multiphase flow rate measurement devices of the gradio-venturi type which, by means of differential pressure measurements distributed along the length of the device, provide information relating to the flow rates of the phases.
A gradio-venturi combines a section that measures a static pressure gradient and a section that measures variation in momentum. That device provides accurate measurement, in particular for the oil-water and oil-gas flows encountered in the oil industry. Nevertheless, it has been observed that measurement accuracy is limited once the proportion of gas by volume exceeds 60% to 70%.
Various systems for measuring multiphase flows are proposed in British patents 1,272,152 and 1,461,537, and in the document 6.2 North Sea Flow Measurement Work shop 1990, National Engineering Laboratory, Glasgow, entitled "Simple full -bore water-cut measurement technique" by D. Brown and JJ. der Boer. In each case, the apparatus described is in the form of an upside-down U-shape and pressure measurements are performed at various heights along each of the branches of the U-shape and then compared to determine the parameters of the flow. Given that none of the measurements relates to changes in dynamic pressure, it is not possible to calculate the individual flow rate of each phase on the basis of those pressure measurements alone.
Patent application WO 93/17305 describes a combination of two gradio-venturis disposed in sections where the effect of gravity makes it possible to measure the density of the mixture. In addition, means are provided for measuring the water fraction and to extract a full sample of the mixture. That system is fully capable of measuring multiphase flows, but it likewise suffers from a limit in terms of measurement accuracy once the volume fraction of the gas exceeds 60% to 70%.
Patent application WO 93/19347 describes a multiphase flow meter associating in series a volume-measuring flow meter and two flow-measuring sensors responsive to the momentum of the measured fluids, and in which the ratio of gas speed to liquid speed is maintained at a constant value, e.g. equal to 1. The volume-measuring flow meter consists, for example, in a turbine or in a flow meter having oval gears. Such an intrusive type of device suffers from well-known drawbacks. In addition, keeping the ratio of gas speed to liquid speed at a constant value constitutes an awkward constraint.
Mention may also be made of the following documents concerning flow rate measurements in nultiphase flows of the oil effluent type: SPE publication 23065 by Gold et al., entitled "Measurement of multiphase well fluids by positive displacement meter"; SPE publication 25620 by Cary et al., entitled "New well testing technology: a portable three-phase test unit"; patent application WO 93/24811; and U.S. Pat. No. 5,287,752. Finally, mention is made of U.S. Pat. No. 4,397,190 which describes a device adapted to making measurements in production wells.
The present invention seeks to perform flow rate measurements on multiphase fluids, and more particularly on oilwell effluents, which may have a gas content exceeding 80% or even 90%.